Cooling apparatus based on heat energy bound to working fluid in phase transition

ABSTRACT

The invention relates to a device for removing heat from an electronic component mounted on a circuit board. The device uses a working fluid circulated through cooling elements which are integrated into a metal matrix composite structure. The cooling elements are positioned and arranged within the structure to efficiently manage heat dissipation by evacuating the heat from the component in a multi-directional manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cooling apparatus based on the principle ofusing a working liquid to remove heat energy.

The need for cooling electronic components is an old and known problem,the significance of which is becoming more pronounced with increasingpower and integration densities, since all electronic componentsgenerate heat, which has to be dissipated to accomplish an optimal andreliable operation of the components. As the field of electronicscontinuously progress in a direction where increasingly high powers areprocessed in increasingly small volumes, the dissipation of heat fromcomponents has become a decisive planning criterion. Many currentelectronic appliances need a cooling capacity that cannot be achieved byconventional metallic cooling fins.

2. Description of Related Art

New methods, such as heat pipes, have recently emerged along withconventional convection cooling. A heat pipe transfers heat veryefficiently; its effective thermal conductivity is of the order of 1000times that of copper. A heat pipe transfers heat from one end to theother as latent heat of phase transition, i.e. a liquid boils andevaporates in the hot end (an evaporator) of the pipe, and the createdpressure difference makes the vapour move to the other, cold end (acondenser) of the pipe, where the vapour emits its latent heat andreturns as a liquid to the evaporator, driven, for example, by capillaryforce. Heat pipes have been commercially available since 1960's.Basically they can respond to the heating needs of electronics, but theindustry has only recently begun to regard heat pipes as a reliable andadvantageous solution for the cooling problems of the most demandingelectronics applications.

Until now, heat pipes have generally transferred heat directly from ahot component to the cooling apparatus by each component having aseparate heat pipe. It is extremely difficult, space-consuming andclumsy to place the heat pipes efficiently using this principal,particularly if there are several components requiring cooling on thesame circuit board. In prior art solutions the heat pipes are generallyeither at the circuit board level, in which case the condensers of thepipes are cooled by heat sinks or the like on the sides of the board, orvertically against the circuit boards, in which case the heat sink isparallel to the circuit board.

U.S. Pat. No. 5,527,588 presents a cooling apparatus for cooling a heatsource using heat energy bound to working fluid in phase transition. Thecooling apparatus comprises a plurality of elongated cooling elementsextending parallel in at least two levels so as to form atwo-dimensional matrix of cooling elements. In addition, the coolingapparatus comprises a heat conducting material binding the coolingelements. In this cooling apparatus the cooling elements are used totransfer heat in two directions, parallel to the level of the coolingelement matrix.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a cooling apparatus forcooling electronic components which is superior to the ones used untilnow and by which the problems of placing heat pipes can be avoided. Anexemplary embodiment of the invention provides a cooling apparatuscomprising elongated cooling elements containing a working fluid. Theelongated cooling elements extend in at least two different directionsin order to form a matrix of cooling elements. The apparatus alsocomprises a heat conducting material configured for binding together thematrix of cooling elements.

The invention is based on multi-directional heat pipes integrated into ametal matrix composite structure.

The cooling apparatus of the invention is characterized in that theelongated cooling elements are arranged parallel to the circuit boardlevel and in at least one direction deviating from the circuit boardlevel.

The invention can significantly improve the cooling of thermallycritical components and thus enable the implementation of more and moreefficient electronic systems and/or reduce the costs caused byconventional cooling systems.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described in greater detail bymeans of examples with reference to the accompanying drawings, in which

FIG. 1 shows a separate heat pipe and the structure thereof,

FIG. 2 schematically shows a cooling apparatus of an embodiment of theinvention,

FIG. 3 shows the principal of a second embodiment of the invention, and

FIG. 4 is the cross section of a cooling apparatus of a third embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the structure and function of a heat pipe 1, known per se,to be used in the invention. The heat pipe is shown partly in crosssection. The heat pipe, which is a gas tight tube-like air-evacuatedcooling element, has two ends. The object to be cooled, or in this casean end 2 that is near a heat generating electronic circuit, is called anevaporator and an opposite end 3 is called a condenser. The heat pipecontains liquid working fluid which can be, for example, ammonia, water,acetone or methanol depending on the application. In the evaporator end2 a liquid medium evaporates into gas and binds evaporation heat, orlatent heat, characteristic of the medium to itself. Evaporation causesa pressure gradient in the pipe forcing the vapour to flow along amiddle part 4 of the pipe towards the condenser 3. The vapour isadiabatically conveyed, whereby the variations in pressure andtemperature are small. In the condenser the vapour condenses back intoliquid, conveying the evaporation heat to the heat sinks of thecondenser. The working fluid is returned in liquid form to theevaporator along a porous prime coat 5 forming the external thread ofthe heat pipe by capillary force. The basic function of a heat pipe isknown in the prior art and will therefore not be explained here ingreater detail.

Alternatively micro heat pipes can preferably be used in the coolingapparatus of the invention in which case the capillary force is causedby the geometry of the pipe. The basic function of a micro heat pipe isalso known in the art and will not be described here in greater detail.

FIG. 2 shows an embodiment of the cooling apparatus based on workingfluid in phase transition of the invention. In accordance with theinvention the cooling apparatus is formed from a matrix of elongatedcooling elements i.e. heat pipes. The circuit board level is marked withthe letter A. In this example the heat pipes are two-dimensional; some 6a extending in parallel from a component 7 parallel to the circuit boardlevel A, and others 6 b vertically against the circuit board level.Corresponding main directions of the cooling effect are marked in theFigure. A heat conductive material 8, into which the heat pipes areembedded sees to it that heat emission from the component takes place atleast to some extent in three dimensions. The heat simultaneously movesas efficiently as possible from the component to the heat pipe throughthe heat conductive material 8.

Deviating from the case in FIG. 2, the vertical heat pipes 6 b canalternatively be at another angle, other than vertical to the circuitboard level A. Alternatively the heat pipes 6 a may be arranged toextend in various directions.

A suitable material for binding heat pipes is metal matrix compositepreferably formed from silicon carbide (SiC) into which the heat pipesare placed and into which molten aluminum is brought for binding thestructure and for making the appearance compact, using, for example, aconventional die casting method. The portion of silicon carbide can be,for example, 70% and the portion of aluminum 30% by weight. Also othermaterials such as silicon nitride, aluminum oxide etc. can be used buttheir heat conductivity is inferior to that of silicon carbide.

The circuit board may comprise one or several components requiringcooling. When the heat pipes have been integrated into a composite, thecooling apparatus according to the invention becomes a fixed structurewhich is easy to mount, space-saving, and durable. The invention can beutilized particularly in the cooling of power electronics, e.g.processors, ASIC circuits, power sources and the components of RFelectronics.

According to the principal shown in FIG. 3 the heat pipes 9 a- 9 f canalso be arranged to form a three dimensional network around thecomponent 7 to be cooled, in which case lost heat can be evenly dividedinto a cooling surface area as wide as possible. Thus, the coolingeffect takes place mainly in all five directions shown by the heat pipes9 a- 9 f presented in rough outline in the Figure or in their desiredsubset according to the space and need of the heat pipes. The circuitboard level is marked with the letter A.

In the case shown in FIG. 3 the cooling effect can be extended to takeplace in six directions if two cooling apparatuses according to FIG. 3are used and the component to be cooled is placed between the two.

As a third alternative, the location of the heat pipes at the circuitboard level vertically against each other so as the heat emission fromthe component takes place parallel to the circuit board level in twodimensions can be mentioned.

In the embodiment in FIG. 4 cavities 10 a- 10 c corresponding with thetopology of a circuit board 12 and components 13 a, 13 b, 13 c thereinare worked into the cooling apparatus 10. The cooling apparatus 10 isthen preferably of the same size as the circuit board 12. Then the heatpipes 11 can be accurately imposed in the immediate vicinity of the hotcomponents and the number and direction of the heat pipes can be variedaccording to what is most appropriate. By using very small so-calledmicro heat pipes, the heat pipes can be placed at desired locations ofthe cooling apparatus as areas of desired sizes, with no need to planhow to place individual pipes.

The heat pipes can be built to either transfer heat to external coolingfins 16 or only to balance the temperature distribution of the circuitboard metal matrix combination 12, 15 by conveying heat from the hotcomponents 13 a, 13 b, 13 c to the other material. Cooling the actualcircuit board 12 is in some cases also preferable since up to 70% of thelost heat of the components can vanish into it. In this case the circuitboard can be used as a heat sink and its temperature distributionbecomes homogeneous.

Such a worked structure can be formed to be very compact particularly ifthere are several components to be cooled on the circuit board in whichcase the cooling apparatus is easy to handle and install and thecircuits are also easy to maintain. A thermal grease 14 or anothermedium is preferably placed between the components and the coolingapparatus to improve thermal contact between the components and thecooling apparatus.

It is obvious to one skilled in the art that the different embodimentsof the invention are not restricted to the examples described above butcan vary within the scope of the claims below.

What is claimed is:
 1. A cooling apparatus including a circulatingworking fluid, the circulating working fluid transitioning in phase todissipate heat generated by at least one electronic component insertedin a circuit board, the cooling apparatus comprising: a plurality ofelongated cooling elements configured to contain the circulating workingfluid, the plurality of cooling elements forming a matrix of coolingelements; and a heat conducting material binding the matrix of thecooling elements, wherein (i) at least one of the elements of theplurality of elements is arranged to extend parallel to a plane formedby the circuit board, (ii) at least one of the elements of the pluralityof elements is arranged to extend parallel to the plane formed by thecircuit board but not parallel to the elements of (i), and (iii) atleast one element of the plurality of elements extends in a directionnot parallel to the plane formed by the circuit board.
 2. The coolingapparatus of claim 1, further comprising a cavity formed on an exteriorsurface of the apparatus, the cavity facing the circuit board andsubstantially conforming to a shape of the at least one electroniccomponent.
 3. The cooling apparatus of claim 2, further comprising amedium configured to improve thermal contact is placed between the atleast one component and the cooling apparatus.